1. Field of the Invention
The present invention relates to a torque motor driving device for wire cut electrical discharge machines that drives a torque motor with an AC power source.
2. Description of the Related Art
A torque motor is best suited for feeding or winding the wire electrode of a wire cut electrical discharge machine. In a wire cut electrical discharge machine, an appropriate torque is generally applied to the shaft to which the wire electrode bobbin is attached, in the direction opposite to the wire electrode feed direction, to prevent the wire electrode from being loosened. In known use of a torque motor in a wire cut electrical discharge machine described in, for example, Japanese Patent Application Publication No. 7-60552, a used wire electrode is not stored in a recovery box, and a wire electrode recovery bobbin driven by a torque motor is disposed on the wire electrode feed unit, so that the wire electrode can be recovered efficiently while the tension is kept constant by controlling the torque motor.
The wire electrode is generally 0.02 mm (minimum wire electrode diameter) to 0.40 mm (maximum wire electrode diameter) in diameter. A thick wire electrode requires a large torque because the bobbin is large and heavy and the inertia is large. A thin wire electrode requires a small torque because the bobbin is small and light, the inertia is small, and a torque large enough to break the wire electrode cannot be applied. As described above, the output torque of a torque motor needs to be adjusted properly depending on the diameter of a wire electrode to be used.
A torque motor is a type of induction motor and, when rotating so as to be pulled in a direction opposite to that of a torque generating on the output shaft, the effective current flowing through the motor is approximately proportional to the torque generating on the output shaft, regardless of the number of revolutions. Accordingly, to obtain a desired torque, the AC voltage to be applied to the motor needs to be changed so that the current corresponding to the torque flows. To generate torques corresponding to the range from the maximum wire electrode diameter to the minimum wire electrode diameter described above, the effective voltage needs to be changed to a value from approximately 5% to 100% of the rated voltage of the motor.
As a circuit that changes the AC voltage of a torque motor, a driving circuit adopting the resistor voltage divider method was used conventionally. In recent years, however, a driving circuit adopting the triac method has been used.
FIG. 6 schematically shows a torque motor driving circuit by the resistor voltage divider method. The resistor voltage divider method divides the power voltage V1 of an AC power source 10 using resistors 51, 52, 53, 54, and 55 and a single-phase torque motor 20, and performs switching using relays 61, 62, 63, 64, and 65, so that unnecessary voltages are applied to the resistors. In the resistor voltage divider method, it is necessary to prepare the resistors and relays required to apply a set voltage across the single-phase torque motor 20. Because power is consumed by the resistors, a plurality of large-size power resistors are necessary, thereby increasing the device size. In addition, problems are that cost is high, a large loss of power is caused, and the output torque of the single-phase torque motor 20 changes depending on the power voltage V1 of an AC power source 10.
FIG. 7 schematically shows a torque motor driving circuit by the triac method. The triac method obtains a desired torque by changing the firing angle of a triac 68 to change the effective voltage to be applied to the single-phase torque motor 20. The triac method can reduce a loss of power and make a current to be applied to a torque motor coincide with an instructed value even in an area where a different power voltage is used by feeding back a detected current value to a firing angle control circuit (not shown). However, since control by this method is limited to the commercial frequency of the AC power source 10, especially when a low torque is required, the firing angle becomes low and, as shown in FIG. 9, the ratio of the OFF time of the voltage to be applied to the single-phase torque motor 20 becomes much larger than that of the ON time. As a result, fluctuations in a current flowing through the single-phase torque motor 20 become large as shown in FIG. 9B and the number of torque fluctuations for each turn increases.
To suppress torque fluctuations in the triac method, an inverter can be used to change the number of revolutions of the motor. An inverter is generally used to control the number of revolutions of an AC motor. AC power is converted into DC power by a converter and a voltage to be applied to the motor is converted into AC power again by a bridge circuit including semiconductor switches. The so-called PWM control is allowed, in which the effective frequency of a motor is changed by determining the rotation frequency from several hertz to tens of hertz and the switching frequency of tens of kilohertz and changing the duty at which the semiconductor switch is turned ON and OFF within the switching frequency.
FIG. 8 schematically shows a torque motor driving circuit by the inverter method. The torque motor driving circuit by the inverter method full-wave rectifies the AC voltage of the AC power source 10 using a diode bridge converter including a first diode 11, a second diode 12, a third diode 13, and a fourth diode 14 and then smoothes and converts the rectified voltage into a DC voltage using an inductor 70 and a high-capacitance electrolytic capacitor 72. The DC voltage is converted into an AC voltage again by an inverter including a first FET 15, a second FET 16, a third FET 17, and a fourth FET 18, and the AC voltage is applied to the single-phase torque motor 20.
A PWM signal generating circuit 76 changes the ON/OFF duty of the PWM signal to make the detected current value that a means (not shown) obtains by detecting the current flowing through the motor coincide with the instructed current value, so that the current value matches the instructed current value and the desired torque can be obtained even when power fluctuations occur as shown in FIG. 10 or even in an area in which a different power voltage (such as 200 V or 220 V) is used. In addition, since the OFF time can be significantly reduced than in the triac method, the current flowing through the coil of the single-phase the torque motor 20 becomes continuous and torque fluctuations are reduced.
However, since the voltage to be applied to the single-phase torque motor 20 is the DC voltage converted by the converter, if the switching frequency and the duty of the PWM signal are fixed, voltage fluctuations caused when the polarity of the voltage is reversed become sharp, thereby causing torque fluctuations. Accordingly, a PWM/rotation frequency signal synthesizing circuit 78 has a duty adjusting circuit 79, which adjusts the duty of switching depending on the phase of the rotation frequency so that the current waveform becomes sinusoidal as shown in FIG. 10 to prevent torque fluctuations. This duty adjusting circuit 79 is a complicated circuit that gradually increases the duty so that the phase of the rotation frequency has the minimum value at 0 degrees and the maximum value at 90 degree and reduces the duty so that the phase has the minimum value at 180 degrees. On the other hand, an high-speed FET with a small ON resistance can be used as the semiconductor switch to reduce a loss of power.
Since the resistor voltage divider method or triac method of the prior art has large torque fluctuations and other problems as described above, use of an inverter with less torque fluctuations can be considered. However, this method also has disadvantages. That is, the converter unit that converts AC power into DC power generally uses the high-capacitance electrolytic capacitor 72 (see FIG. 8) to store the electrical energy required to supply the full-wave rectified DC voltage to the motor.
The electrolytic capacitor 72 greatly reduces its capacitance over time and the reduction causes much heat generation due to charge and discharge, possibly causing explosion or liquid leakage. Accordingly, the electrolytic capacitor 72 needs to have a large capacitance. However, a high-capacitance electrolytic capacitor is large and expensive. To ensure long term reliability, it is necessary to give a higher priority to long term reliability by increasing the capacitance or give a higher priority to the size and const by reducing capacitance margins.
For the single-phase torque motor 20, the rated power frequencies are generally 50 Hz and 60 Hz. The rotation frequency can be fixed to 50 Hz or 60 Hz, but a circuit that adjusts the duty of the switching depending on the phase of the rotation frequency is required to obtain a sinusoidal current waveform that prevents torque fluctuations and this circuit is complicated and expensive.